atof() returning float instead of double

Question

I have text files containing numbers with precision upto 12 decimal places.

So to load the number into a c++ vec I wrote this

void txt2vec(const std::string& file, std::vector<double>& data)
{
std::string line;
std::ifstream myfile(file);
if (myfile.is_open())
{
    while ( getline (myfile,line) )
    {

        std::cout << atof(line.c_str()) << std::endl;
        //data.push_back(atof(line.c_str()));
    }
    myfile.close();
}
}

atof should return a double but I am getting float

This is the output of the program

where as it should be

  -0.0340205617249
  -0.0873644948006
   0.078953281045
   0.115022487938
   0.0809852406383
   0.118468873203
   0.11332821846
   0.112745501101
  -0.0331071354449

`ostream::operator<<(double)` prints 6 digits of precision by default. See also `std::setprecision` — Igor Tandetnik, Aug 24 '17 at 15:10
No, you're not getting a `float` result, you're getting a `double` result. — Keith Thompson, Aug 24 '17 at 15:52
This isn't the problem, but do you really need the extra stuff that `std::endl` does? `'\n'` ends a line. — Pete Becker, Aug 24 '17 at 15:54

score 1 · Accepted Answer · answered Aug 24 '17 at 15:17

You are likely getting the expected values, but you are not printing the full precision in your shown code.

The << operator for output streams and doubles has a default precision of 6 for general use and readability. See std::ios_base::precision.

The default precision, as established by std::basic_ios::init, is 6.

If you want to display the full number you need to set the precision with the number of digits a double may contain, e.g. use numeric_limits.

std::cout << -0.0340205617249 << std::endl; // -0.0340206
std::cout.precision(std::numeric_limits<double>::digits10 + 1);
std::cout << -0.0340205617249 << std::endl; // -0.0340205617249

atof() returning float instead of double

1 Answers1